<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    div {
      width: 200px;
      height: 100px;
      border: 10px solid red;
      padding: 20px;
    }
  </style>
</head>
<body>
  <div></div>
  <script>
    var div = document.querySelector('div')

    console.log( div.offsetWidth )  // width + 2 padding + 2 boder = 260

    console.log( div.clientWidth )  // width + 2padding = 240
  </script>
</body>
</html>